1. Field of the Invention
The present invention relates generally to control rod drives for nuclear reactors and particularly to snubbers for such drives which control the speed of control rod drop into the nuclear reactor.
2. Description of the Prior Art
Known prior art control rod drives mount snubber assemblies to the upper extension of a control rod drive housing to decelerate the drop of control rods during reactor shut down. The dropping of the control rods into the reactor is known as a "scram" condition. Such snubber assemblies are mounted above the nuclear reactor when the control rod drive is mounted thereto and depend upon the control rod drive housing being filled with liquid to insure proper snubbing action. An example of such a prior art snubber assembly may be found in U.S. Pat. No. 3,822,439 issued to Robert A. Wallin et. al. Should the control rod drive housing be low on liquid, a dry "scram" condition will occur and the dropping of the control rod will not be properly decelerated causing the leadscrew connected control rod to drop too rapidly damaging both the control rod drive and the nuclear reactor.
The above mentioned prior art snubber assemblies also mount a stacked Belleville washer type buffer spring assembly at the end of the snubber assembly to absorb shock caused by the "scram" condition including dry "scram" conditions. However, such spring assemblies are costly and tend to lose their efficiency with continued use.